Y13

Dates & Time:

• Mondays: 11:40-12:35 

• Tuesdays: 09:35-11:40 

• Thursdays: 08:40-10:30 

• Fridays: 09:35-10:30 

General Guide to Answering Asymmetric Encryption and Digital Signature Questions

Understanding the Concepts

1. Private Key: A secret key used for decryption in asymmetric encryption. It is never shared and has a corresponding public key.

2. Public Key: A key that can be shared publicly and is used for encryption in asymmetric encryption.

3. Digital Signature: A cryptographic value that is calculated from the data and a secret key, used to verify the authenticity and integrity of a message.

Structuring Your Answers

1. Define Key Terms: Start by clearly defining any key terms mentioned in the question.

2. Describe Processes: When asked to describe a process, break it down into clear, logical steps.

3. Explain Usage: Explain how and why certain cryptographic methods are used, focusing on their purpose and benefits.

4. Provide Examples: Where possible, provide examples to illustrate your points.

Example Questions and How to Answer Them

Question 1: Private Key

Question: State what is meant by a private key. Answer:

• Definition: A private key is a secret key that is not shared or transmitted.

• Matching Public Key: It has a corresponding public key.

• Decryption: Used to decrypt data encrypted with its matching public key.

Question 2: Asymmetric Encryption Process

Question: Describe the process of asymmetric encryption. Answer:

• Encryption: The message is encrypted using the recipient’s public key.

• Decryption: The message is decrypted using the recipient’s private key.

Question 3: Digital Signature Verification

Question: Explain how a digital signature is used to verify a message when it is received. Answer:

• Decryption: The message and digital signature are decrypted using the receiver’s private key.

• Signature Decryption: The digital signature is decrypted with the sender’s public key to get the message digest.

• Hashing: The received message is hashed to reproduce the message digest.

• Comparison: Compare the two message digests to check for alterations.

Digital Certificates and Email Security

Question 4: Data Items in a Digital Certificate

Question: Name three data items present in a digital certificate. Answer:

• Serial Number: Unique identifier for the certificate.

• Certificate Authority (CA): The entity that issued the certificate.

• Public Key: The public key of the certificate owner.

Question 5: Issuing a Digital Certificate

Question: State the correct word and justify your choice for the missing words in the method of issuing a digital certificate. Answer:

• (i) Public: The generated public key is sent to the CA.

• (ii) Public: The key and data are encrypted using the CA’s public key.

• (iii) Private: The CA signs the digital document using its private key.

Question 6: Email Security

Question: Explain how Beena can be sure that she has received a message that is authentic and from Alexa. Answer:

• Digital Signature: The name given to the encrypted message digest.

• Verification: Beena can verify the message by using Alexa’s public key from the digital certificate to decrypt the message digest and compare it with the hash of the received message. If they match, the message is authentic and from Alexa.

Tips for Full Marks

• Be Concise: Stick to the key points.

• Use Technical Terms: Correctly use terms like "private key," "public key," "encryption," "decryption," "message digest," and "digital signature."

• Structure Your Answer: Clearly separate each point for clarity.

• Provide Justifications: When asked to justify, explain the reasoning behind your answers.